1. Field of the Invention
This invention relates generally to microwave circuits and, more particularly, to apparatus and method for modifying frequency characteristics of resonant cavities. Although the present invention is discussed with reference to band-pass filters, the technique has application to oscillators, delay lines, filters, etc. operating in the microwave frequency region.
2. Description of the Related Art
In the implementation of microwave circuits, a component in which the resonant frequency characteristics can be conveniently altered is frequently required. For example, the output circuit of the aircraft Traffic Alert and Collision Avoidance System II (TCAS II) differential phase shift keying (DPSK) and pulse modulated transmitter, a band-pass filter in the 1030 MHz region capable of high power operation is required. This filtering is required to reduce the off-channel DPSK spectral components to an acceptable level. In addition, the filter must be a low loss component within the filter pass band because of the expense in generating power in this frequency range.
In the related art, such requirements can be met by the pass band filter illustrated in FIG. 1 and FIG. 2. FIG. 1 shows a perspective view of the resonant cavity with the cover removed, while FIG. 2 shows a cross sectional view of the resonant cavity structure. The resonant cavity 9 is fabricated in a housing 15. Passing through the cavity 9 is the center conductor element 10. The center conductor element 10 passes through the cavity 9 and is positioned in aperture 15A and aperture 15B of the housing 15. The portion of the center conductor element 10 in aperture 15A is held in place by a set screw 18 and finally soldered in the aperture 15A for mechanical and electrical coupling to the housing 15. The portion of the center conductor element extending into aperture 15B has an insulating (i.e., typically teflon) cover thereon. The insulating cover 11 prevents the center conducting resonant element 10 from contacting the housing 15. The aperture 15 is threaded and has a conducting tuning element 21 and locking element 22 inserted therein. The position of the tuning element 21 adjusts the distance 1 between the tuning element 21 and the center conductor element 10. The activating signals are applied to the device by coaxial cable 13. Coaxial cable 13 has center conductor element 13A, a shielding conductor 13B and a dielectric material 13C therebetween. The coaxial cable 13 has a coupling element 13D that is adapted to connect to coupling element 17 attached to the housing 15. The coupling element 17 has a conductor 14 associated therewith that couples the center conductor 13A of coaxial cable 13 with the center conductor element 10. Aperture 16 in the wall of the cavity permits a radiation coupling between adjacent cavities.
The operation of the tunable resonant cavity of the related art shown in FIG. 1 and FIG. 2 can be understood in the following manner. A microwave frequency signal is introduced into the cavity 9 and applied to the center conductor element 10. The signal applied to the center conductor element 10 will typically have a distributed spectral composition. The geometry of the cavity 9, the geometry of the center conductor element 10 and their interrelationship will result in a defined resonant frequency. This resonant frequency will be the dominant frequency of the signal generated by the center conductor element 10. The spacing between the end of the center conductor element 10 in aperture 15B and the tuning element 21 forms a capacitive coupling to the housing 15. By varying the distance between the center conductor element 10 and the tuning element 21 designated by 1 in FIG. 2, the capacitive coupling to the housing 15 can be controlled, consequently controlling the capacitive loading on center conductor element 10. The capacitive loading, in turn, controls the resonant frequency of the resonant structure. The distance between the end of the center conductor element 10 and the tuning element 21 is accomplished by loosening locking element 22, rotating tuning element 21 until the appropriate resonant frequency is obtained and tightening the locking element. The locking element is secured against the tuning member to prevent unwanted changes in the position of the tuning element. However, the forcing of the locking element 22 against the tuning element 21 can result in sufficient movement of the tuning element to provide an unacceptable change in the resonant frequency. Typically, the procedure involves iterative steps until the resonant structure has the desired resonant frequency. In addition, the tuning procedure is relatively complex, requiring loosening of the locking element, positioning of the tuning element and tightening of the locking element. In addition, electric fields can be strong upon application of power to the cavity and these fields can produce voltage breakdown. Finally, the fabrication of the device can be difficult, requiring close tolerances for the fabrication of aperture 15A and aperture 15B, while requiring soldering operation that involves the housing 15.
A need has therefore been felt for apparatus and method that can modify or tune the frequency characteristics of a microwave component, that can be easily fabricated and that can be conveniently adjusted.